I think the concern isn't over particles that are just smaller than our cells; it's over particles that are _significantly_ smaller that our skin cells.
The idea being the particles have to be small enough to diffuse across the cell membrane to be a concern. Something half the size of a cell is going to have a very hard time permeating a cell membrane.
That send, another commenter (far more knowledgable on the topic than I) above points out that these aren't actually nano-scale particles. They are roughly on the same scale as a cell, and thus your point may hold in this context.
The idea being the particles have to be small enough to diffuse across the cell membrane to be a concern. Something half the size of a cell is going to have a very hard time permeating a cell membrane.
That send, another commenter (far more knowledgable on the topic than I) above points out that these aren't actually nano-scale particles. They are roughly on the same scale as a cell, and thus your point may hold in this context.